1. Field of the Invention
The present invention relates to structures comprising metal-organic coordination polymers including frameworks, or “MOF” crystalline materials which contain scintillating structures within each unit cell of the MOF lattice. The invention more particularly relates to a stilbene MOF which generates a light output when irradiated by light and by subatomic particles, such as protons.
Understanding and predicting the photophysical properties of chromophores in the solid state is important for an increasing number of organic materials applications, where control over the spatial interactions of chromophores represents a significant challenge. The geometry of a molecular assembly is often difficult to predict due to the large number of intermolecular forces that can influence the packing of molecules in a disordered network or crystal. Metal-organic frameworks (MOFs) are a class of crystalline coordination polymers with the potential to control these interactions through appropriate choice of the constituent metal and ligand units. MOFs consist of metal ions or clusters connected by organic linker groups, which can lead to structural rigidity, high porosity, and well-defined architectures. These properties are desirable for a variety of applications, and the use of MOFs for gas storage, drug delivery, separations, and catalysis is currently being explored. The structural stability of MOFs results from strong metal-ligand coordination, which can afford some degree of predictability to the framework geometry and leads towards rational methods of crystal engineering. This has been utilized as a strategy to engineer non-centrosymmetric crystals for nonlinear optics (NLO) applications and asymmetric catalysis, for example. This led us to postulate that MOFs could offer predictable, well-defined environments for chromophores in solid-state materials.
Despite the large number of MOF materials described in the literature, however, reports of luminescent MOFs are scarce, especially those that display ligand-based emission. The majority of materials in this class exhibit metal ion-centered luminescence due to the incorporation of lanthanide elements into their framework. One potential advantage of the use of ligand-based emission in MOFs is that it should readily be tunable through variation of the nature of the linker and/or the structure of the framework. Additionally, calculations to regarding the electronic structure of prototypical porous 3-D MOFs have suggested that the bandgaps of these materials can be altered by changing the degree of conjugation in the ligand. Such factors may prove important for the practical application of these materials.
Our interest in the present case is to provide a stilbene-based MOF. Stilbene has a range of technologically important uses. For example, it is an important component in solid-state scintillating materials, as its luminescence can be used to discriminate neutron and gamma-ray radiation. Additionally, stilbenes are commonly employed as a backbone motif in organic NLO materials, and may be considered the fundamental unit of the electro- and photoluminescent conjugated polymer, poly(para-phenylene vinylene) (PPV). Moreover, incorporation of stilbene as the linker into a MOF lattice effectively suppresses the cis-trans isomerization of stilbene, a nonradiative pathway, by fixing the ligand configuration through rigid coordination, affording a material with increased QY and brightness. Furthermore, stilbene excimer luminescence has been used for sensitive detection of DNA and antibody binding events. We are, therefore, interested in probing structural- and guest-dependent luminescence of stilbene-based MOF materials as model systems and for their potential sensing capabilities.
In particular, ionizing particle and radiation detectors represent a critical need for such disparate fields as homeland security, nuclear nonproliferation, medical imaging and therapy, and oil exploration. This is particularly true, for detectors capable of detecting fast neutrons without degradation of the energy and trajectory information, because the presence of these particles is an indication of fissile elements including weapons-grade nuclear material.
Existing particle detection technologies, however, suffer from several problems that limit their ability to be used for applications such as high-volume cargo screening and portable radiation detection. In particular, because neutrons are uncharged particles they do not ionize matter as they pass through it and, therefore, cannot be detected directly. Instead, neutrons are detected by the signals produced by secondary particles, typically recoil nuclei or energetic ions produced by capture. For example fast neutrons are usually detected as an incoming neutron strikes a target hydrogen atom producing a recoil proton within a scintillation material, which is coupled to an electronic detector such as a photomultiplier.
Many sensor materials can be used to convert neutrons for detection; but each is disadvantaged in some way. Capture reactions require moderation of the neutron energies, destroying all information of trajectory and energy distributions. Furthermore commercially available particle detectors using gas-based, 3He, and BF3. These require large volumes of gas at substantial pressures in order to provide adequate sensitivity. These materials also tend to be expensive due to complex manufacturing processes and/or high material costs and some of them, such as boron trifluoride, require special handling due to their poisonous and corrosive nature.
Currently fast neutron scintillation detectors include plastic scintillators and organic liquids based on aromatic small molecules such as toluene and pseudocumene. These conventional organic scintillators are limited by the low light output signal generated from ions as compared with the output generated by gamma and beta radiation. This is caused by the high rate of energy loss (linear energy transfer or LET) by high mass particles resulting in higher rates of non-radiative recombination, and results in a relatively high threshold energy for discrimination of neutrons from background gamma radiation. The open pore structures in MOFs lead to low mass density and therefore reduced LET relative to conventional materials.
2. Related Art
Current methods for overcoming some of the shortcomings of the prior art include using a porous medium doped with a fissionable material for capturing neutrons and incorporating a rare earth material therein that fluoresces when stimulated by fission fragments resulting from the capture reaction. For example Hiller, et al., (U.S. Pat. No. 5,973,328) and Wallace, et al., (U.S. Pat. No. 6,876,711) describe a neutron detector comprised of a fissionable material contained within a glass film fabricated using a sol-gel method. When the glass film is bombarded with neutrons, the fissionable material emits fission particles and electrons that are detected using standard UV and particle detection methods. Dai, et al., (U.S. Pat. No. 7,105,832) describe a composite scintillator for neutron detection comprising a matrix material fabricated from an inorganic sol-gel precursor solution homogeneously doped with a liquid scintillating material and a neutron absorbing material.